Fix leak in mkfs_msdos(..) by initializing img to NULL and free'ing at the end
of the function

Differential Revision: https://reviews.freebsd.org/D4405
MFC after: 1 week
PR: 204943
Reviewed by: emaste, jilles
Reported by: David Binderman <dcb314@hotmail.com>
Sponsored by: EMC / Isilon Storage Division